When operating a semiconductor device which converts optical signals to electrical signals, such as an optically activated switch or an optical detector, one wants to achieve the most efficient optical to electrical conversion possible. In the devices like an optically initiated switch, a strong switch turn-on is generally desirable. Some types of optical switches rely on an avalanche effect to activate them.
To achieve these ends, it is necessary for the device to sustain large voltages or, as it is sometimes referred to, have a high voltage stand-off. High voltages generally translate into more efficient optical to electrical conversions and to stronger switch turn-on. High voltages (generally in the range of several kilovolts) are a requirement to achieve avalanche in non-linear, optically initiated switches.
Where high voltage stand-off is desirable, it is also desirable that the electrode spacing between anode and cathode be sufficiently small so that carrier transit times and/or electromagnetic wave propagation times are short. This results in a fast response time. A preferred electrode gap may range from tens of microns to several millimeters. Unfortunately, high stand-off voltages are usually incompatible with such small gaps between electrodes. Devices with close electrode spacing often have poor voltage hold-off characteristics. One cause of this deficiency is due to high field points in, under, and/or around the electrode metallization which leads to high carrier injection into the semiconductor. As discussed in the article "Contact and Metallization Problems in GaAs Integrated Circuits," Journal of Vacuum Science and Technology, A 4(6), Nov./Dec. 1986 pp. 3085-3090 by N. Braslau, the assumption of perfect planar Schottky or ohmic contacts often cannot predict the measured current-voltage (I-V) characteristics of a device. Braslau found that a theoretical model which included high-field defect points at the contact could explain many experimental observations. These are random microscopic defects that can produce localized high field regions. In fact, a lower contact resistance was experimentally correlated with a higher density of defects at the contacts. In the classes of devices considered here, a good barrier at the contacts is actually desirable so that large voltages can be applied without large dark currents. High field points caused by structural, chemical, or other physical states tend to produce a poor barrier. Because such defects are difficult to control during fabrication, it is common to see surface photoconductors that are similar in geometry, metallization, and material, yet have vastly different I-V characteristics. For example, it is not uncommon to have two similar devices fabricated by the same process, one of which will hold-off high voltage while the other will not.
Such defects may also affect the operation of bulk photoconductors, such as the bulk avalanche semiconductor switch (BASS) described in U.S. Pat. Nos. 4,891,815, 4,218,618, 4,438,331 and 4,782,222. Such devices often cannot be used with a DC high voltage bias because of their poor voltage hold-off characteristics. For this reason, pulsed power supplies, which are expensive and bulky, must be used instead with BASS devices.
A method used to increase voltage stand-off and reduce device dark current in semiconductor photodectors is to fabricate special barrier enhancement layers of certain materials under the metallization in order to effectively increase the so-called barrier height of the contact/semiconductor junction. A discussion and example of such special engineering and fabrication is given by J. I. Chyi et al. in "Low dark current and high linearity InGaAs MSM photodetectors," Electronic Letters, Vol. 30, p. 355, Feb. 17, 1994. This approach requires highly sophisticated engineering and fabrication techniques which entail considerable expense.
From this brief background, it would clearly be desirable if a device with poor voltage stand-off characteristics could be modified to have good voltage stand-off characteristics. This invention provides an inexpensive and simple way for imparting good stand-off characteristics.